I think this is an exciting opportunity for applications where simulation speed is an issue, offering the prospect of massive performance gains.
As far as I understand from the v-rep documentation, the interface with the physics engines is designed with modularity in mind,
so that, at least in theory, they should be easily interchangeable.

Can you give me some advice as to how to start if I was to fiddle with the physics engines in vrep?
or is there any specific plan to make use of bullet 3's gpu acceleration features in the near future?

it is true that we will soon have to update the Bullet source code to match the last developments. If most functions have stayed same, then it shouldn't be too difficult, it is just important to take great care about the parts of the Bullet source code that we modified, otherwise something will break.

What has to be done is simply take the dynamicsPlugin project and update the Bullet folder. Then adjust here and there.

this was not yet updated. What we will first do, is allow for a mechanism to run several versions of Bullet in the same V-REP, in oder to avoid situations where the Bullet update breaks some compatibility with older scenes.

Bullet now has improved physics algorithms designed specifically for robotics which improve the accuracy during interactions such as grasping. In addition to the new python API there is a C and C++ API. https://github.com/bulletphysics/bullet3/releases

cool! They're also very interested in API feedback at this early stage. If there is anything that would make V-REP integration easier you may wish to comment with a github issue. They are incorporating some new ik and constrained opti algorithms for control as well.

GPU acceleration is only partially supported, since this only gives conditionally better results (typically with particles, or many isolated rigid body groups, which is rarely the case with a robot simulator).